Process for the production of silicon tetrafluoride

ABSTRACT

A continuous process is disclosed for the production, with a high yield, of high purity silicon tetrafluoride, starting from an aqueous solution of fluorosilicic acid, which comprises: 
     (a) reacting an aqueous solution of fluorosilicic acid with concentrated sulphuric acid, carried out with a short residence time and preferably under fluido-dynamic conditions of turbulence, inside a reaction zone; 
     (b) separating a gaseous stream containing silicon tetrafluoride from a liquid stream of aqueous sulphuric acid, carried out inside a separation zone, directly connected with the reaction zone; 
     (c) washing said gaseous stream with concentrated and cold sulphuric acid carried out inside a washing zone, for the purpose of separating a gaseous stream of purified silicon tetrafluoride; and, possibly 
     (d) further purifying said gaseous stream of purified silicon tetrafluoride by means of techniques of adsorption on an adsorbent solid material.

The present invention relates to a continuous process for theproduction, with a high yield, of high purity silicon tetrafluoride,starting from an aqueous solution of fluorosilicic acid.

Silicon tetrafluoride is an important intermediate, useful for theproduction of valuable products, such as pure silica, silanes, puresilicon for solar cells, silicon nitride for ceramic products andfluorinated carbon-silicon polymers for materials for architecturaluses. For better information, reference is made to the specification ofthe following patents: U.S. Pat. No. 3,969,485; U.S. Pat. No. 4,374,111;U.S. Pat. No. 4,442,082; U.S. Pat. No. 4,458,087; and Japan 59174506.

In the art, processes are known for producing hydrofluoric acid andsilicon tetrafluoride, starting from aqueous fluorosilicic acid, such asdisclosed, e.g., in the following patents: U.S. Pat. No. 3,969,485; U.S.Pat. No. 3,218,124; and U.S. Pat. No. 4,062,930.

According to such processes of the prior art, the reaction is carriedout inside a stirred reactor, to which a stream of concentratedsulphuric acid, or oleum, and a concentrated aqueous solution offluorosilicic acid is fed, for developing hydrofluoric acid and silicontetrafluoride in the form of a gas stream, which is submitted to awashing step with concentrated sulphuric acid. An undesiredcharacteristic common to these processes of the prior art is constitutedby the low yields of conversion of fluorosilicic acid into silicontetrafluoride, above all due to phenomena of hydrolysis, leading to theformation of silica.

In U.S. Pat. No. 4,470,959, a process is disclosed, according to which astream of aqueous fluorosilicic acid is fed to an intermediate pointbetween the head and the bottom of a vertical tower, and a stream ofconcentrated sulphuric acid is fed near the head of the tower, and fromthe head of the tower an overhead gas stream containing silicontetrafluoride is recovered, and from the bottom of the tower a stream ofdiluted sulphuric acid is recovered.

By means of this process, yields of useful reaction product within therange of 95.4% to 98.5% are achieved, with a content of hydrofluoricacid in the so-produced silicon tetrafluoride, being lower than 0.1% byvolume (equivalent to about 200 ppm by weight).

In general, the aqueous fluorosilicic acid used in the processes ofproduction of silicon tetrafluoride, comes from the units of fluorineseparation of the facilities of production of phosphoric acid by the wetprocess. These aqueous solutions contain several impurities, and, inparticular, compounds of boron, phosphorus and arsenic elements whichremain, even if in trace amounts, in the silicon tetrafluoride.Eliminating such impurities, which are highly harmful, in particularwhen silicon tetrafluoride is used for the production of elementalsilicon of solar or semiconductor grade, is a technical problem.

On the basis of such a state of the prior art, the need was felt forhaving available a simple and cheap process for preparing silicontetrafluoride free, or substantially free from impurities, andespecially from those impurities which endanger its use in theproduction of elemental silicon of solar or semiconductor grade,obtained continuously and in improved yields.

Such a need is fulfilled by means of the process of the presentinvention, according to which:

(a) an aqueous solution of fluorosilicic acid, at a concentration of atleast 8% (w/w) and sulphuric acid at a concentration of at least 90%(w/w) are reacted with each other, a preferably operating underturbulent conditions, and with a contact time longer than 1 second,inside a self-maintaining reaction zone at a temperature of at least 90°C., and under a pressure slightly higher than ambient pressure, toproduce gaseous silicon tetrafluoride;

(b) the gaseous stream containing silicon tetrafluoride is separatedfrom a liquid stream of diluted aqueous sulphuric acid, by operating ata temperature of at least 70° C. inside a separation zone, directlyconnected with the reaction zone;

(c) the gaseous stream is washed with cold sulphuric acid having aconcentration of at least 96% (w/w), inside a washing zone, for thepurpose of producing a gaseous stream of purified silicon tetrafluoride;and, optionally

(d) the gaseous stream of purified silicon tetrafluoride is furthermorepurified by being contacted with an absorbent solid material.

The sole FIGURE of the Drawing represents a flow diagram of the processaccording to the invention.

By conducting the process hereinabove within the range of otherconditions as hereunder specified, conversion yields of fluorosilicicacid into silicon tetrafluoride are achieved, which are equal to, orhigher than, 99% (as expressed as Si), whereby silicon tetrafluoride isobtained having a content of hydrofluoric acid lower than 35 ppm (w/w)and a content of boron, phosphorus and arsenic respectively lower than1, 10 and 4 ppb (w/w), suitable.

Step (a)

In (a) step of the process of the present invention, a stream of aqueousfluorosilicic acid and a stream of concentrated sulphuric acid areindependently fed to a reaction zone, wherein they are brought intocontact, and caused to react with each other.

Normally, the aqueous solution of fluorosilicic acid containsfluorosilicic acid at a concentration within the range of from 8 to 30%(w/w), and preferably of from 12 to 20% (w/w). This aqueous solution cancome from the fluorine removal units of the facilities for theproduction of phosphoric acid by the wet process. In this case, thesolution contains several impurities, in particular boron, phosphorusand arsenic compounds.

The concentrated sulphuric acid used in the process of the presentinvention has a concentration within the range of from 90 to 100% (w/w),and preferably in the order of from 96 to 99% (w/w).

The two streams are continuously fed to a tubular reactor, inside whicha fluido-dynamic regime of turbulence is preferably maintained. Thestreams are preferably not pre-heated, and the temperature which isestablished in the reactor depends essentially on the water balanceinside the system. However, under the above shown conditions, thetemperature is at least 90° C., and is generally within the range offrom 90° to 120° C. The reaction is furthermore carried out under apressure slightly higher than ambient pressure, e.g., under a relativepressure higher than 30 kPa, and generally 40 to 50 kPa, and with aresidence time of approximately 10 seconds.

Under these conditions, an instantaneous and optimum mixing of thereactants occurs, and a substantially complete conversion offluorosilicic acid into gaseous hydrofluoric acid and gaseous silicontetrafluoride is achieved, according to the equation: ##STR1##

An advantage of the process of the present invention is the fact thataqueous solutions also containing a low level of fluorosilicic acid canbe used, with the phenomena of hydrolysis, leading to the formation ofsilica, according to the equation: ##STR2## being avoided.

The so-obtained reaction mixture is directly sent to a separation zone,directly connected with the reaction zone.

Step (b)

In (b) step of the process of the present invention, a gaseous stream,containing silicon tetrafluoride, is separated from a liquid stream ofdiluted acqueous sulphuric acid.

This separation can be carried out inside any equipment for gas/liquidseparation, such as, e.g., a packed tower, a tray tower, a spray tower,a film tower, and so forth.

In a preferred embodiment, the tubular reactor, inside which thereaction of the step (a) is carried out, is positioned on the top of andcoaxially with the separation tower of step (b) and the reaction productflows directly from the reaction zone to the upper portion of theseparation zone.

Furthermore, inside the separation zone, the separation process iscarried out at a temperature of from 70° to 120° C., and under a slightrelative pressure (of the order of 1.5 kPa), causing a gaseous stream ofsilicon tetrafluoride containing hydrofluoric acid, plus trace amountsof water vapour to be separated, at a temperature equal to or slightlyhigher than the reactor temperature. This gaseous stream, which isgenerated in the upper portion of the separation zone, is sent to thewashing with sulphuric acid in the step (c).

Furthermore, from the bottom of the separation zone, sulphuric acid isrecovered, at a concentration which can vary within the range of from 60to 90% (w/w), and preferably from 78 to 82% (w/w), which contains tracesof fluorine ions, essentially present as dissolved hydrofluoric acid, inamounts which are typically of the order of 0.3-0.9% (w/w). This acidcan be directly used as the acid for the chemical attack of phosphaticrocks in the production of phosphoric acid by the wet process.Alternatively it can undergo regeneration treatments.

Step (c)

According to the process of the present invention, the gaseous streamproduced in the (b) step is submitted to a washing with cold,concentrated sulphuric acid in a washing zone.

For that purpose, a washing tower can be used, which ensures a goodgas-liquid contact, e.g., a packed tower, a tray tower, and the like.

The gaseous stream coming from the (b) step, which contains, besidessilicon tetrafluoride, hydrofluoric acid and water vapour, is fed to apoint close to the bottom of the washing zone, at a temperature equal toor nearly equal to the temperature at the outlet from the separationzone.

At a point close to the top of the washing zone sulphuric acid is fed,which has a concentration of preferably at least 96% (w/w), pre-cooled,e.g., at a temperature of 10° C. or less, and anyway at a temperaturehigher than the freezing point of the same acid.

By operating under the above-disclosed conditions, from the bottom ofthe washing zone a stream of sulphuric acid is drawn, which has aconcentration typically of the order of 94-96% (w/w), and with a contentof fluorine ions typically lower than 1% (w/w).

The control of the temperature in the washing zone can be advantageouslyaccomplished by recycling a portion of said bottom stream, after apreliminary cooling inside a heat exchanger.

Also this stream of sulphuric acid can be directly used for the chemicalattack of phosphatic rocks, or, as an alternative, it can undergoregeneration.

Furthermore, by operating within the range of the above indicatedconditions, at the top of the washing zone a stream of purified gaseoussilicon tetrafluoride is produced, which contains, in any case, lessthan 35 ppm (w/w) of hydrofluoric acid.

Step (d)

According to the process of the present invention, the stream ofpurified silicon tetrafluoride can be submitted to a furtherpurification, for the purpose of completely, or substantiallycompletely, eliminating, possible impurities, such as boron, phosphorusand arsenic compounds.

This purification is carried out by making the gaseous stream flow overan adsorbed solid material, and in particular over activated charcoal.

By this treatment, the content of arsenic is reduced from a typicalvalue of 2-5 ppm (w/w) to less than 4 ppb (w/w), this last value beingthe limit of detectability utilizing standard analytical technique.

By means of this treatment, a silicon tetrafluoride is obtained, whichis suitable for preparing elemental silicon for specialized uses such asin solar cells and in electronics, such as in semiconductors.

According to a preferred embodiment of the present invention, powdersilica is introduced together with the aqueous solution of fluorosilicicacid.

In this way, the amount of hydrofluoric acid is reduced, due to theoccurrence of the reactions: ##STR3##

The following experimental example is illustrative but not limitative ofthe present invention.

EXAMPLE

The process is carried out by means of a continuous pilot plant, with aproduction potentiality of approximately 0.3 kg/hour of silicontetrafluoride, which is schematically shown in the Figure of the drawingtable.

In this Figure, there are indicated: with (3) the tubular reactor; with(4) the separation tower, packed with Raschig rings; and with (7), thewashing tower filled with Raschig rings. These pieces of equipment, aswell as the connection lines, are made from a corrosionresistantmaterial (teflon; polyvinylidene fluoride, etc.).

In particular, referring to the Figure to the reactor (3) 2.48 kg/hourof an aqueous solution of fluorosilicic acid at a concentration of 14.9%(w/w) (line 2) and 8.89 kg/hour of sulphuric acid at 99% (w/w) (line 1)are fed.

The aqueous fluorosilicic acid contains the following impurities: boron,1 ppm (w/w), arsenic, 15 ppm (w/w) and phosphorus, 0.3% w/w.Furthermore, the temperature of the fed streams is of about 20° C.

Inside reactor (3), the reaction is self-maintained at the temperatureof approximately 120° C., under a relative pressure of approximately 50kPa, with a contact time of about 9 seconds.

The reaction mixture passes directly from the reactor (3) to theunderlying separation tower (4), operating under a relative pressure ofabout 1.3 kPa; from said tower (4), a liquid bottom stream (line 5) isseparated, at the temperature of approximately 90° C., which isconstituted by sulphuric acid at a concentration of 81% (w/w), and witha content of hydrofluoric acid of about 0.55% (w/w) and of silicon ofabout 25 ppm (w/w), and at the top of tower 4 (line 6) a gaseous streamat the temperature of 110° C. develops, which contains silicontetrafluoride, hydrofluoric acid and traces of water vapour, which isfed to the bottom of the washing tower (7). To the top of said tower (7)(line 12) a stream is fed, which consists of 2.0 kg/hour of sulphuricacid at a concentration of 99% (w/w), cooled to about 12° C. in the heatexchanger 13.

From the bottom of tower (7) (line 8) a liquid stream is discharged,which has a concentration of 96.8% (w/w), and contains 0.2% (w/w) ofhydrofluoric acid.

A portion of this stream is recycled to the tower (7), through the line(9), after a preliminary passage through the heat exchanger (10), forcooling it to approximately 12° C.

The remaining portion of the bottom stream is discharged through theline (11).

The stream of gaseous silicon tetrafluoride coming from the tower (7),containing 3 ppm (w/w) of arsenic, is conveyed, by means of the line(14), to the tower 15, packed with granules of activated charcoal.

From the tower 15, through the line (16), 0.32 kg/hour is recovered ofsilicon tetrafluoride, with a yield higher than 99%.

This silicon tetrafluoride, at the mass spectrometry analysis, shows thefollowing impurities: hydrofluoric acid, lower than 20 ppm (w/w); boron,lower than 1 ppb (w/w); phosphorus, lower than 10 ppb (w/w); andarsenic, lower than 4 ppb (w/w).

We claim:
 1. A process for the preparation of silicon tetrafluoridecomprising:(a) contacting a 12-20% (w/w) aqueous solution offluorosilicic acid with 90-100% (w/w) of an aqueous solution ofsulphuric acid at a contact time of at least 1 second, in a reactionzone, said reaction zone operating at a temperature of from 90° C. to120° C. and under a pressure greater than 30 kPa, said contactingresulting in the liberation of a gaseous stream of silicontetrafluoride; (b) separating said gaseous stream of silicontetrafluoride from a liquid stream of a diluted aqueous solution ofsulphuric acid produced in a separation zone, said separation zoneoperating at a temperature of from 70° C. to 120° C. and connecteddirectly with and positioned below said reaction zone; (c) washing saidgaseous stream of silicon tetrafluoride with cold sulphuric acid havng aconcentration of at least 96% (w/w), in a washing zone; said coldsulphuric acid having a temperature higher than the freezing point ofsaid sulphuric acid to 10° C., and (d) contacting said stream of gaseoussilicon tetrafluoride with activated charcoal.
 2. The process accordingto claim 1, wherein in said (b) step said separation is carried out at atemperature of from 70° to 120° C., under pressure, inside equipment forgas/liquid separation.
 3. The process according to claim 2, wherein thereaction zone of step (a) is positioned at the top of and coaxial withthe separation zone of step (b).
 4. A process for the preparation ofsilicon tetrafluoride comprising:(a) contacting an 8-30% (w/w) aqueoussolution of fluorosilicic acid with 90-100% (w/w) of an aqueous solutionof sulphuric acid at a contact time of greater than 1 second, in areaction zone, said reaction zone operating at a temperature of from 90°C. to 120° C. and under a pressure greater than 30 kPa, said contactingresulting in the liberation of a gaseous stream of silicontetrafluoride; (b) separating said gaseous stream of silicontetrafluoride from a liquid stream of a diluted aqueous solution ofsulphuric acid produced in a separation zone, said separation zoneoperating at a temperature of from 70° C. to 120° C. and connecteddirectly with said reaction zone; (c) washing said gaseous stream ofsilicon tetrafluoride with cold sulphuric acid having a concentration ofat least 96% (w/w), in a washing zone; and (d) contacting said stream ofgaseous silicon tetrafluoride with activated charcoal.
 5. The processaccording to claim 4, wherein in said (a) step, said concentration offluorosilicic acid is within the range of from 12 to 20% (w/w), and saidconcentration of sulphuric acid is within the range of from 96 to 99%(w/w), said reaction is carried out inside a tubular reactor underturbulent conditions and for a contact time of approximately 10 seconds.6. The process according to claim 5, wherein said tubular reactor ofsaid (a) step is positioned at the top of, and coaxially with, theseparation tower of said (b) step.
 7. The process according to claim 4,wherein in said (c) step said washing is carried out with said coldsulphuric acid having a concentration of about 96% (w/w), and precooledat a temperature of about higher than the freezing point of saidsulfuric acid to 10° C.